HORN-STEUCTURES IN THE UNGULATA. 139 



each other, They all, in fact, belong to what has been 

 called the '' Exo-skeleton," which is developed from the 

 skin, while the horn cores of the Bovidse and the completed 

 bony antlers of the stag are, I believe, developments from 

 the ''Endo-skeleton." 



In all true horns, then, I consider that the skin-structnre 

 and the bone-strncture are combined, although the horn of 

 the rhinoceros would appear to be an exception, owing to 

 the almost entire absence of development of the bony part 

 and the often large and heavy horn being entirely produced 

 by the activity of the skin organ, which not very distantly 

 resembles the organ which in the "Right" whales develops 

 the baleen from the skin of the mouth. In both cases there 

 appears to be not only a growth of hair-like fibres but also 

 of a supporting horny cement (if I may call it so) in which 

 the hairs are lodged, and which in the whales forms also 

 a covering of the baleen plate. If we examine the concave 

 base of a rhinoceros horn we may see that the surface is 

 covered with small pores, which during life lodge minute 

 papillae of the skin, from which are developed the hair-like 

 fibres of which the horn mainly consists. 



The mode of growth of the prong-buck's horn, as de- 

 scribed by Mr. Caton, and afterwards observed by Mr. 

 Bartlett at the Regent's Park Gardens, seems quite unique. 

 Instead of all the new growth taking place at the base of 

 the horn, as in all others of the Bovidse, in this antelope it 

 appears that the annual new formation begins at the top and 

 gradually proceeds downwards until the root is reached ; 

 and this different mode of growth appears to be connected 

 with the annual sheddino- of the old horn-sheaths, which 

 are gradually pushed off by the new ones as they are formed 

 inside the old ones from the skin covering the cores. This 

 acrogenous mode of growth, as I have said above, seems 



